1. Field of the Invention
This invention relates to composite fibers comprising heterogeneous composition of an ethylene-vinyl alcohol and a polyester, having high functionalities and aesthetic feeling, and also to the process for producing them.
2. Description of the Prior Art
Polyester fibers are being produced and consumed on a very big scale, thanks to their general-purpose characteristics such as excellent strength and modulus, abrasion resistance, chemical resistance, weather resistance and dimensional stability, which are far superior to those of natural fibers. However, on the other hand, the polyester fibers are, in the field of end-uses of fabrics and clothing which should have high-grade feeling, still inferior to natural fibers in aesthetic feeling and/or high-grade feeling in spite of many efforts made so far to improve the shape of the filament, the structure of the yarn and the like. Furthermore, the polyester fibers still have the following soiling or dirting problems: they are inferior to cotton in the darkening of white cloth, which is a problem of soil redeposition; they are liable to be oil-soiled; their coated products such as polyurethane-coated fabric suffer a problem of color transfer due to migration of disperse dye; and the like. Although the above-mentioned problems of polyester fibers had long been intensively studied, it has been found that such polyester fibers have no or, if any, very small amount of hydrophilic groups, or are modified by copolymerization to only a very small extent or only at the ends of molecules thereof cannot fully solve the above problems. It has also been found that introduction of too large an amount of hydrophilic groups would impair inherent properties of the fiber substrate to make the fiber unusable for the practical purpose and that modification of polymer simply by copolymerization or the like has only limited effect.
Study on why natural fibers such as cotton, silk and wool have excellent hand and aesthetic appearance, or high resistance to soiling has clarified that the natural fibers all have hydrophilic groups to thereby exhibit superior features in the following way when they are processed by using water.
All the natural fibers swell upon absorption of water. Then, the single filaments swell to thicken by about 30 percent in apparent sizes and, also, yarns comprising the filaments will become still thicker due to minute deformation of filaments upon swelling, e.g. crimping of wool by bilateral structure, distortion of cotton by convolution, nonuniform waving of silk, etc., thereby bending and fixing the texture or stitch. If, a fabric comprising such yarn is then dried, the apparent thickness attained upon the swelling now decreases to assure clearances between the filaments, while the texture or stitch is still fixed. Consequently, the contact pressure between the filaments and between the crossing yarns is decreased, and any restricting force therefore will not work when the fabric is deformed by bending, shear, elongation or recovery therefrom to thereby decrease hysteresis loss. This fact gives the fabric larger resilience and liveliness.
It has also been found that the problem of darkening by soil redeposition at washing or soiling by sublimation and migration of disperse dye can markedly be improved by coating the surface of polyester filaments with a hydrophilic polymer.
The present inventors have, taking the above points into consideration, aimed at application of ethylene-vinyl alcohol copolymer to polyester fibers. The ethylene-vinyl alcohol copolymer can, since it swells by absorption of water and has hydrophilic groups, solve the above-described problem of oil dirting or darkening by soil redeposition at washing, and be free from the problem of soiling by sublimation and migration of disperse dye, which problems are inherent to polyester fibers. The present invention is achieved by pursuing and clarifying how to make up ethylene-vinyl alcohol copolymer and polyester into a fiber which can make use of the features of the two.
Japanese Patent Publication No. 5223/1971 discloses a shaped article of polyester comprising ethylene-vinyl alcohol copolymer, which is a hydroscopic polymer, homogeneously mixed therewith to improve the static property of polyester.
However, fibers having a homogeneous blend structure of polyester component and ethylene-vinyl alcohol copolymer component give woven fabrics or knitted fabric being short of bulk and having poor hand, as compared to fibers of heterogeneous blend structure. In the course of study to pursue the reason of this, it was found that the fiber having a homogeneous blend structure shrinks uniformly and deforms only little when immersed in high-temperature hot water.
On the other hand, it was found that in the case of a fiber of heterogeneous blend structure minute deformations generate at various parts, some part bending and some part distorting, when such fiber is immersed in high-temperature hot water. The reason is considered to be that since ethylene-vinyl alcohol copolymer, which swells by absorption of water, is present at localized parts in the cross section of a fiber, strain by swelling will give minute deformations at localized parts in the fiber, which fact then leads to improvement in the bulk and "taste" of an aggregate of the fibers. This is quite similar to the behavior of natural fibers in which minute deformations generate upon swelling.